If you are British, you can stay in Thailand for 30 days without a visa, so it looks like you will need one. This is from the Foreign & Commonwealth Office site:
"Visas
British passport holders are given entry for 30 days, without a visa, on arrival. Overstaying without the proper authority is a serious matter and you can be held in detention until a fine is paid. The only legal way of obtaining a new visa, entry permit or extension of stay is from a Royal Thai Embassy or Consulate, an Immigration Officer at a point of entry into Thailand or one of the Immigration Offices around the country.
Visas issued by visa shops, travel agents or by any other means are likely to be illegal and lead to criminal proceedings, which may result in a large fine or imprisonment. You would also be prohibited from entering Thailand again. Entry to Thailand is normally refused if you have a passport with less than six months validity.
Single parents or other adults travelling alone with children should be aware that some countries require documentary evidence of parental responsibility before allowing lone parents to enter the country, or, in some cases, before permitting the children to leave the country. Thai authorities do not require such evidence, but if you have any concerns please check with Thai representation in the UK."